This invention relates to washing machines and, more particularly, to washing machines including switched reluctance motors.
Consumers who purchase washing machines for laundering clothing have become more sophisticated. Consumers expect a washing machine to operate quietly and efficiently. The washing machine must be reliable and have little or no maintenance costs. The washing machine preferably has a high capacity to size ratio. In other words, consumers would like the ability to launder a large amount of clothing in a small amount of time without needing a large, commercial-sized washing machine. Consumers also expect the washing machine to be gentle on their clothing.
Both vertical axis and horizontal axis washing machines are currently available in the marketplace. Vertical axis washing machines include a washing tub with an axis that is substantially vertical or at a slight angle with respect to vertical. In contrast, horizontal axis washing machines include a washing tub with an axis that is substantially horizontal or at a slight angle with respect to horizontal.
Due to significant improvements in recent years, the demand for horizontal axis washing machines is on the rise. Horizontal axis washing machines typically utilize less power and a reduced amount of water and/or detergent than vertical axis washing machines. Therefore, the operating costs that are associated with horizontal axis washing machines are typically lower than with vertical axis washing machines. In addition, the more efficient tumbling action that is imparted to clothing in a horizontal axis washing machine generally provides improved stain removal and/or a shorter washing cycle that is more gentle on clothing.
While there are significant structural differences between these two types of washing machines, both generally include a cabinet shell and a tub assembly. The tub assembly includes an outer tub that is suspended in the cabinet shell and an inner tub that is rotatably mounted within the outer tub. The inner tub typically includes fins that project radially inwardly from the annular side wall of the inner tub. The fins impart movement to the washing fluid and the articles of clothing that are located in the inner tub. Annular side walls of the inner and outer tubs are concentric. A pivotable door that is secured to the cabinet shell provides access to the inner tub to load or unload laundry. In vertical washing machines, the pivotable door is typically located on the top side of the cabinet shell. In horizontal washing machines, the pivotable door is typically located on the front side of the cabinet shell.
In operation, clothes are loaded into the inner tub. A mixture of water, detergent and/or other washing fluids is pumped into the inner tub. A motor rotates a drive assembly that, in turn, rotates the inner tub typically using a reciprocal or rotational movement. The reciprocal and/or rotational movement cleans the clothes. As can be appreciated, the motor of the washing machine has a significant impact on the capacity, reliability, efficiency operating noise and other operating characteristics of the washing machine. Improvements that are made to the motor will help manufacturers meet or exceed consumer demands for these product features.
Reluctance motors have typically been used in washing machines. Reluctance motors produce torque as a result of the rotor tending to rotate to a position that maximizes the inductance of an energized winding of the stator. A drive circuit generates a set of stator winding currents that are output to the stator pole windings and that set up a magnetic field. In response to the magnetic field, the rotor also rotates in an attempt to minimize the reluctance of the magnetic circuit (and to maximize the inductance of the energized winding of the stator). In synchronous reluctance motors, the windings are energized at a controlled frequency. In switched reluctance motors, control circuitry and/or transducers are provided for detecting the angular position of the rotor. A drive circuit energizes the stator windings as a function of the sensed rotor position. The design and operation of switched reluctance motors is known in the art and is discussed in T. J. E. Miller, xe2x80x9cSwitched Reluctance Electric Motors and Their Controlxe2x80x9d, Magna Physics publishing and Clarendon Press, Oxford, 1993, which is hereby incorporated by reference.
In switched reluctance motors, there are two distinct approaches for detecting the angular rotor position. In a xe2x80x9csensedxe2x80x9d approach, an external physical sensor senses the angular position of the rotor. For example, a rotor position tranducer (RPT) with a hall effect sensor or an optical sensor physically senses the angular position of the rotor. In a xe2x80x9csensorlessxe2x80x9d approach, electronics that are associated with the drive circuit derive the angular rotor position without an external physical sensor. Angular rotor position can be derived by measuring the back electromotive force (EMF) or inductance in unenergized windings, by introducing diagnostic pulses into energized and/or unenergized windings and sensing the resulting electrical response, or by sensing other electrical parameters and deriving the angular position of the rotor.
The stator of conventional switched reluctance motors generally includes a solid stator core or a laminated stator with a plurality of circular stator plates. The stator plates are punched from a magnetically conducting material and are stacked together. The solid core or the stack of stator plates define salient stator poles that project radially inward and inter-polar slots that are located between the adjacent stator poles. Winding wire is wound around the stator poles. Increasing the number of winding turns and the slot fill increases the torque density of the electric machine. The stator poles of switched reluctance motors typically have parallel sides that do not inherently hold the winding wire in position. Tangs on radially inner ends of the stator poles have been provided to help maintain the winding wire on the stator poles with some limited success. Tangs limit an area between radially inner ends of the stator poles, which may cause problems during the winding process.
In switched reluctance motors using the xe2x80x9csensedxe2x80x9d approach, a rotor position transducer (xe2x80x9cRPTxe2x80x9d) is used to detect the angular position of the rotor with respect to the stator. The RPT provides an angular position signal to the drive circuit that energizes the windings of the switched reluctance motor. The RPT typically includes a sensor board with one or more sensors and a shutter that is coupled to and rotates with the shaft of the rotor. The shutter includes a plurality of shutter teeth that pass through optical sensors as the rotor rotates.
Because rotor position information is critical to proper operation of a switched reluctance motor, sophisticated alignment techniques are used to ensure that the sensor board of the RPT is properly positioned with respect to the housing and the stator. Misalignment of the sensor board is known to degrade the performance of the electric motor. Unfortunately, utilization of these complex alignment techniques increases the manufacturing costs for switched reluctance motors equipped with RPTs.
The RPTs also increase the overall size of the switched reluctance motor, which can adversely impact motor and product packaging requirements. The costs of the RPTs often place switched reluctance motors at a competitive disadvantage in applications that are suitable for open-loop induction electric motors that do not require RPTs.
Another drawback with RPTs involves field servicing of the switched reluctance motors. Specifically, wear elements, such as the bearings, located within the enclosed rotor housing may need to be repaired or replaced. To reach the wear elements, an end shield must be removed from the housing. Because alignment of the sensor board is critical, replacement of the end shield often requires the use of complex realignment techniques. When the service technician improperly performs the alignment techniques, the motor""s performance is adversely impacted.
In an effort to eliminate the RPTs and to reduce manufacturing costs and misalignment problems, the xe2x80x9csensorlessxe2x80x9d approach for sensing rotor position is used. The various methods of performing the xe2x80x9csensorlessxe2x80x9d approach have drawbacks that are attributable, in part, to variations in the inductance and resistance of the stator windings due to assembly and tolerance variations.
Washing machines incorporating switched reluctance motors can be improved in several important areas. Specifically, it is desirable to improve the torque density of switched reluctance motors that are used in washing machines. By increasing the torque density of the motor, the size of the motor can be reduced for a given torque density and/or the size can be maintained with an increase in torque density. As a result, the size of the inner and outer tubs can be increased for a given application and/or the size of the washing machine can be reduced for a given capacity.
It is also desirable to eliminate the need for RPTs in switched reluctance motors that are used in washing machines. It is also desirable to assemble the stator of a switched reluctance motor in a highly uniform and repeatable manner to improve the performance of sensorless switched reluctance motors by reducing variations in the inductance and resistance of the stator. As a result, the xe2x80x9csensorlessxe2x80x9d methods of sensing rotor position will be improved.
A washing machine according to the invention includes a tub that is rotated by a switched reluctance motor. The switched reluctance motor includes a shaft that rotates the tub. The switched reluctance motor includes a stator with a plurality of circumferentially-spaced stator segment assemblies each with a stator segment core. Winding wire is wound around the stator segment core. A rotor is connected to the shaft and defines a plurality of rotor poles. The rotor tends to rotate relative to the stator to maximize the inductance of an energized winding. A drive circuit energizes the winding wire around the stator segment assemblies based on a rotational position of the rotor.
In other features of the invention, the switched reluctance motor directly drives the tub. The rotor is connected to a shaft that is connected to the tub. Alternately, the switched reluctance motor is connected to a drive assembly that includes a spinner pulley that is connected to the tub. The spinner pulley is rotated by a belt that is driven by the shaft that is connected to the rotor. The tub assembly includes an outer tub and an inner tub that is mounted for rotation concentrically in the outer tub.
According to other features of the invention, each stator plate has an outer rim section and a tooth-shaped pole section. The end cap assembly includes a pair of end caps that are secured to opposite ends of the stator segment core and a pair of retainer sections interconnecting the end caps on opposite sides of the stator segment core. The end cap assembly defines an annular retention channel within which the winding wire is wound. The retention channel facilitates improved precision in the winding process and tends to reduce winding creep during use.
The washing machine according to the present invention includes the switched reluctance motor with improved torque density. The torque output of the switched reluctance motor can be increased for increased washing capacity. Alternatively the motor dimensions can be reduced for a given washing capacity to reduce the weight and the dimensions of the washing machine. In addition, the stator segment assemblies can be manufactured with greater uniformity and with lower variations in inductance and resistance. As a result, sensorless rotor position sensing techniques can be employed more readily, which dramatically lowers the manufacturing costs of the switched reluctance motor and improves the reliability of the motor in the field.
Other objects, features and advantages will be apparent from the specification, the claims and the drawings.